1. Field of the Invention
The present invention relates to an adaptive equalizing transversal filter of a reduced electric power consumption for use in communication system, and more specifically, to an adaptive equalizing transversal filter having reduced power consumption. Particular utility of the present invention is for use in communication system for equalizing a transmission waveform which is subject to deformity because of communication line characteristics, and other sources.
2. Description of Related Art
Referring to FIG. 1, there is shown a block diagram of one example of a prior art adaptive equalizing transversal filter. The shown adaptive equalizing transversal filter is of (N)th order. Data sampled and digitized by an analog-to-digital converter is supplied to an input "INPUT" of the transversal filter, which includes a number of delay circuits ("T") 1A, 1B, 1C, . . . , 1L, 1M and 1N, which are cascaded as shown, and each of which is formed by, for example, a register. Each of the registers 1A, 1B, 1C, . . . , 1L, 1M and 1N are driven by a clock not shown, so that in response to each clock signal, each of the registers 1A, 1B, 1C, . . . , 1L, 1M and 1N latches an output of a preceding register. Therefore, the input signal is sequentially shifted through the registers 1A, 1B, 1C, . . . , 1L, 1M and 1N. An output of each of the registers 1A, 1B, 1C, . . . , 1L, 1M and 1N is connected to an input of an associated multiplier 2A, 2B, 2C, . . . , 2L, 2M and 2N, which is also connected to receive an output of an associated multiplication coefficient generator ("Co") 3A, 3B, 3C, . . . , 3L, 3M and 3N, so that the output of each register 1A, 1B, 1C, . . . , 1L, 1M and 1N is multiplied with a coefficient outputted from the associated multiplication coefficient generator 3A, 3B, 3C, . . . , 3L, 3M and 3N. Outputs of all the multipliers 2A, 2B, 2C, . . . , 2L, 2M and 2N are supplied to an adder (".SIGMA.") 4. An output of the adder 4 constitutes an output of the transversal filter, and is connected to a quantizing discriminator ("slicer") 5, so that the output of the adder 4 is quantized and allocated to an expected value "OUTPUT". The output of the adder 4 and the output of the quantizing discriminator 5 are supplied to a subtracter ("SUB") 6, which generates an error signal "ERROR" composed of a difference between the output of the adder 4 and the output of the quantizing discriminator 5. This error signal "ERROR" is supplied to each of the multiplication coefficient generators 3A, 3B, 3C, . . . , 3L, 3M and 3N for respective order places of the transversal filter.
As shown in FIG. 2, each of the multiplication coefficient generators 3A, 3B, 3C, . . . , 3L, 3M and 3N for determining the coefficients of the respective order places of the transversal filter, comprises a multiplier 8 receiving a signal f(k) from the register (1A, 1B, 1C, . . . , 1L, 1M and 1N) of the corresponding order, the error signal "ERROR" and a given constant .alpha., and a subtracter 9 receiving at a subtrahend input an output of the multiplier 8 and at a minuend input an output of a register ("T") 10 having an input connected to an output of the subtracter 9. Thus, the subtracter 9 subtracts the result of the multiplication of the signal f(k), the error signal "ERROR" and the given constant .alpha., from the output of the register 10, namely, the output of the subtracter 9 before one timing (namely, one clock). The subtracter 9 outputs the result of the subtraction, as the multiplication coefficient, to the associated multiplier 2A, 2B, 2C, . . . , ,2L, 2M and 2N.
By way of example, assume that the input signal "INPUT" is f(k) at a timing "k", and the order of the transversal filter is "m". Therefore, At the timing "k", the output of the register 1A becomes f(k), and the output of the register 1B becomes f(k-1). The output of the register 1M becomes f(k-(m-1)), and the output of the register 1N becomes f(k-m). In addition, assume that the multiplication coefficient of respective orders (namely, the output of respective multiplication coefficient generators 3A, 3B, 3C, . . . , 3L, 3M and 3N) is "a(m, k)". Under this condition, an equalized signal g(k) outputted from the adder 4 is expressed as follows: ##EQU1##
This equalized signal g(k) is supplied to the quantizing discriminator 5, so that the equalized signal g(k) is quantized and allocated to the expected value which is here expressed by "s(k)". Accordingly, the error signal outputted from the subtracter 6, which is expressed by "e(k)", is expressed by e(k)=s(k)-g(k). In automatic adaptive equalizing, the circuit operates to minimize this error "e(k)".
For example, in FSLE (Fractionally Space Linear Equalizer), the multiplication coefficients of the transversal filer are automatically and sequentially updated in accordance with this error, for the purpose of minimizing the error. Accordingly, the updating sequence can be expressed by EQU a(m, k+1)=a(m, k)-.alpha..times.e(k).times.f(m, k)
Here, ".alpha." is called a "step size", which is a fixed constant value as mentioned above.
As seen from the above, in the prior art adaptive transversal filter, the updating of the multiplication coefficients is executed at each clock pulse, and therefore, the multiplication of {.alpha..times.e(k).times.f(m, k)} is repeated at each clock pulse . However, the multiplication of {.alpha..times.e(k).times.f(m, k)} requires large power consumption, and therefore, the prior art adaptive transversal filter requires large power consumption.
For a general understanding, reference can be made to J. J. Werner, "TUTORIAL ON CARRIERLESS AM/PM-PART I-FUNDAMENTALS AND DIGITAL CAP TRANSMITTER", Communication to ANSI X3T9.5, TP/PMD Working Group, Minneapolis, Jun. 23, 1992 and U.S. Pat. No. 4,247,940, the contents of which are incorporated by reference in its entirety into this application. Werner's paper proposes to modify the CAP transmitter with a transversal filter. However, if the system was designed in accordance with Werner's paper, the system needs a large power supply, similar to the above mentioned prior art example, and in addition, becomes large in scale.